The course of the operation process in the column of a liquid chromatograph is temperature dependent, e.g., the retention time may vary by 3 to 4% per degree Celsius temperature variation. The retention time is defined as the dwell duration of the substance in the separation column, which dwell duration is necessary for identifying the substance. Thus, on the one hand it is possible to use the column temperature as a parameter for optimizing the separation process. On the other hand, however, it is necessary that this temperature is kept constant at a defined value over the whole longitudinal extent of the column.
There are substantially three well known methods for keeping the temperature in the separation column constant. The most common method is to house the column within an air convection thermal regulator,the inner air temperature of which is controlled (see e.g. Hewlett-Packard Technical Information Bulletin entitled "High-Speed Liquid Chromatograph Model 1010A", April, 1973). Another method is to mount the separation column within an aluminum body, the temperature of which is controlled (see e.g. Varian Product Bulletin SEP-2182A entitled "5000 Series Liquid Chromatographs", USA 20M 978). According to the third method the separation column is concentrically mounted within a tube through which a liquid such as water flows, which liquid is kept at constant temperature by means of a liquid convection thermal regulator (i.e., Snyder and J. J. Kirkland, Introduction to Modern Liquid Chromatography, page 128, John Wiley & Sons, 1974).
In any of the aforementioned arrangements substantial axial and radial temperature gradients can occur in the separation column, since the mobile phase enters the column at ambient temperature and is heated to column temperature there. The required amount of heat must be supplied through the colum wall which usually consists of stainless steel. The transmission of heat from the column wall to the mobile phase, and the thermal conductivity of the mobile phase flowing laminarly through the stationary phase are not sufficient to assimilate the raised temperature present at the column entrance. This causes the above mentioned temperature gradient in the separation column. For example, in an air thermal regulator at a temperature of 80.degree. C., temperature differences up to 20.degree. C. were measured between column entrance and column exit at a column of 25 cm length and 4.6 mm inner diameter when 4 ml/min of water was flowing through it. Since the temperature gradients are dependent on the type of the column and the position thereof within the thermal regulator, consistency of the analysis result is difficult to achieve. Moreover, the separation capability of the column is substantially affected by the temperature gradients.
In order to avoid these drawbacks it has already been proposed to heat the mobile phase before it enters the separation column (CZ-Chemie-Technik 1 (1972) pp. 73-78). According to this proposal, the heat required for the heat exchanger is taken from the ambient air of the thermal regulator. Due to a poor heat transmission this requires a relatively large heat exchanger volume of about 0.6 ml. The development of liquid chromatography, however, has led to short and small volume separation columns which often are operated with solvent gradients, i.e., the composition of the mobile phase varies during separations. Additionally, it is desired to heat not only the solvent but also the sample substance, i.e., the heat exchanger should be located between the sample injection device and the column entrance. In both aforementioned cases, however, a relatively large heat exchanger volume is not tolerable. It must not be biffer than a few microliters.